Rotor construction for alternating current induction motor

ABSTRACT

The present invention pertains to an improved rotor bar design and method of fabrication of asynchronous motors using such rotor bars. The rotor bar design employs &#34;T&#34;-shaped rotor bars that are interleaved around a magnetic core forming a rotor having a minimum amount of bonding interfaces between the rotor bars and end ring portions. The &#34;T&#34;-shaped rotor bars may either be formed from a single piece or from multiple pieces of copper. The single piece rotor bar method provides N bonding interfaces per end ring, where N is the number of rotor bars used. Where the &#34;T&#34;-shaped rotor bars are formed from three pieces for a rotor having N rotor bars, there will be 2N bonding interfaces for each end ring. Also, the &#34;T&#34; shape of the rotor bars may be added after straight rotor bars have been installed in the rotor bar channels in the magnetic core with their end portions extending beyond the rotor bar channels. In addition, high-speed operation is facilitated by the addition of a shrink-fit beryllium-copper containment ring on each end ring. This containment ring acts to contain the copper end rings against outward forces developed during operation of the motor while having a similar coefficient of thermal expansion to copper (the end ring material) so that significant thermally derived differential forces are not developed between the end rings and their containment rings.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed generally to the field of squirrelcage-type asynchronous motors such as alternating current inductionmotors. More particularly, the present invention is directed to a rotorbar design and method of assembly of the rotor bars which results in theends of the rotor bars interleaving to form rotor end rings.

2. The Prior Art

High-speed, high efficiency induction motors such as those for use inelectrically powered vehicles and other applications require strong,highly conductive end rings. Previous lower efficiency designs requiredresistive end rings in order to develop start-up torque and/or simplifycontrol. Copper is generally recognized as the material of choice forfabricating such highly conductive end rings, but its materialproperties pose two major difficulties. First, the large number ofbonded interfaces (brazed, soldered or welded) between the individualrotor bars reduces the overall electrical conductivity, and second, thephysical strength of copper metal, being a relatively soft and ductilematerial, is not adequate for long endurance high-speed operation.

One previous assembly method, as disclosed in U.S. Pat. No. 4,131,990issued Jan. 2, 1979 to Roach, utilized cast end ring assemblies tofacilitate rotor assembly. According to this method, rotor bars areinserted through the stack of rotor laminations which form the magneticcore of the motor with end portions of the rotor bars protruding beyondthe end laminations at opposite ends of the magnetic core. A pair of endrings disposed at opposite ends of the core are joined to the protrudingend portions of the rotor bars. The axially facing inner end surface ofeach end ring has a series of arcuately spaced radially extendingchannels formed therein with intervening arcuately spaced radiallyextending ribs defined therebetween, with the channels receiving theprotruding end portions of the rotor bars therein, and with the ribsbearing tightly against the end laminations at opposite ends of thelaminated core to maintain the laminated core in tightly compressedcondition. While fit for its intended purpose, this method presents avery large number of surfaces which need to be successfully bonded forgood electrical conductivity throughout the end rings. A furtherdrawback of the method is the requirement for specially cast end ringassemblies.

U.S. Pat. No. 5,283,941 issued Feb. 8, 1994 to Meyer et al. describes amethod for brazing rotor bars to end rings of an asynchronous AC motorwhich employs treated copper end rings and copper rotor bars in aprocess that allows brazing without temperature extremes affecting themechanical properties of the pre-treated copper. While fit for itsintended purpose, a drawback of this method is the limited nature of therotor bar to end ring electrical contact which will result in a higherresistance in the end ring than is desired for certain applications.

U.S. Pat. No. 4,970,424 issued Nov. 13, 1990 to Nakamura et al.discloses a high speed induction motor using aluminum rotor bars andaluminum end rings with an iron containment/balancing ring around eachcast aluminum end ring. As pointed out in the patent, aluminum end ringssuffer from operation at high speed and tend to fracture, hence the needfor an iron containment ring about them. While fit for its intendedpurpose, the end rings formed of aluminum have less electricalconductivity than end rings formed from copper. Copper has asignificantly greater coefficient of thermal expansion than iron and,accordingly a structure having copper end rings with an iron containmentcould not be easily manufactured as there would be risk of fracture ofthe containment at high operating temperatures or during themanufacturing process itself.

SUMMARY OF THE INVENTION

The present invention is directed to an improved rotor bar design andmethod of fabrication of asynchronous motors using such rotor bars.According to a first aspect of the invention, arcuately spaced apartrotor bars which pass through rotor bar channels formed of rotor barslots in individual laminations of the magnetic core of the motorparallel to the rotational axis thereof are formed of an elongateportion having a constant cross section parallel to the magnetic corelaminations and include two portions protruding beyond the endlaminations at opposite ends of the magnetic core. The protrudingportions are, at one end, a head much like the head of a "T", and at theother end, a straight continuation of the elongate portion. Thus therotor bars are "T-shaped" having the head of a "T" at one end (the headend) and a straight portion at the other end (the end portion). Therotor bars are formed to fit closely with one another in an interleavingarrangement. The T-shaped rotor bars are inserted through the magneticcore in an alternating fashion so that the head end of one "T" is alwaysadjacent and essentially in contact with the end portions of twoadjacent (clockwise and counterclockwise) "T"s, and so on. In thismanner the opposing end rings are formed of the alternating head end andend portions of the T-shaped rotor bars. The end ring assembly is thensubjected to a process such as brazing, soldering or welding to completethe physical and electrical connection of all of the heads and endportions of the T-shaped rotor bars at the end rings. In this way, for arotor having N rotor bars, there will only be N braze joints in each endring. As each braze joint increases the resistance of the end ring, aminimum number of such braze joints is desirable in order to maximizeefficiency and minimize losses thus increasing the output power levelthat the motor is capable of sustaining. The T-shaped rotor bars mayeither be initially formed as T-shaped rotor bars from a single piece ofcopper as by casting, stamping, machining, or the like, or assembledfrom multiple pieces of copper which are brazed, soldered, welded, orthe like, to form the T-shaped rotor bars. Where the T-shaped rotor barsare formed from three pieces (i.e., two additional braze joints perrotor bar) for a rotor having N rotor bars, there will be only 2N brazejoints in each end ring. While more than the previously describedversion, this is still an improvement over the prior art.

According to a second aspect of the present invention, the "T" shape ofthe rotor bars may be added after straight rotor bars have beeninstalled in the rotor bar channels in the magnetic core with their endportions extending beyond the rotor bar channels. Such a fabricationtechnique may be realized in a number of ways. According to one method,small pieces of copper which mate to the shape of the sides of thestraight rotor bars may be inserted between each pair of rotor bars soas to form the end ring. These may be tack welded or soldered in placeto hold them prior to end ring consolidation step. In this case, thereare twice as many brazed joints in the end ring as when preformed singlepiece "T"-shaped rotor bars are used, but this is still an advantageover prior art structures.

According to a third aspect of the present invention, high-speedoperation is facilitated by the addition of a shrink-fitberyllium-copper containment ring on each end ring. This containmentring acts to contain the copper end rings against outward forcesdeveloped during operation of the motor while having a similarcoefficient of thermal expansion to copper (the end ring material) sothat significant thermally derived differential forces are not developedduring operation between the end rings and their containment rings.

OBJECTS AND ADVANTAGES OF THE INVENTION

Accordingly, it is an object of the present invention to provide animproved asynchronous alternating current motor and method offabrication.

It is a further object of the present invention to provide anasynchronous alternating current motor capable of high efficiencyoperation.

It is a further object of the present invention to provide anasynchronous alternating current motor capable of high-speed operation.

It is a further object of the present invention to provide anasynchronous alternating current motor and method of fabrication whichreduces the production costs of fabrication over existing asynchronousalternating current motors.

It is a further object of the present invention to provide anasynchronous alternating current motor and method of fabrication whicheliminates the need for separately formed end ring assemblies thusreducing the production costs of fabrication over existing asynchronousalternating current motors.

It is a further object of the present invention to provide a rotor foran asynchronous alternating current motor and method of fabricationwhich yields a rotor that has lower electrical resistance and losses andhence higher electrical operating efficiency, that produces less wasteheat in operation, and that is capable of higher sustained power outputin operation.

Yet a further object of the present invention to provide an asynchronousalternating current motor and method of fabrication which provides amore reliable motor with fewer bonds subject to cracking and otherwell-known failure modes.

These and many other objects and advantages of the present inventionwill become apparent to those of ordinary skill in the art from aconsideration of the drawings and ensuing description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of an end magnetic core lamination of anasynchronous alternating current motor showing an axial view of a singleT-shaped rotor bar which is one of a plurality of such rotor bars whichare used to form the rotor end rings according to a preferred embodimentof the present invention.

FIG. 2 is a front top perspective view of the magnetic core laminationstack of an asynchronous alternating current motor according to apreferred embodiment of the present invention with the rotor barsremoved for clarity.

FIG. 3 is an end view of one of the T-shaped rotor bars of anasynchronous alternating current motor according to a preferredembodiment of the present invention.

FIG. 4 is a top view taken along lines 4--4 of FIG. 3 of one of theT-shaped rotor bars of an asynchronous alternating current motoraccording to a preferred embodiment of the present invention.

FIG. 5 is a side view of the magnetic core lamination stack of anasynchronous alternating current motor according to a preferredembodiment of the present invention with only two adjacent rotor barsshown (and not inserted in the stack) for clarity.

FIG. 6 is a side view of the magnetic core lamination stack of anasynchronous alternating current motor according to a preferredembodiment of the present invention with all rotor bars in place andsurrounding by containment rings.

FIG. 7 is a top view of the rotor of an asynchronous alternating currentmotor according to a preferred embodiment of the present inventionshowing how the rotor bars interleave to form an end ring surrounding bya containment ring.

FIG. 8 is a top perspective view of a portion of the magnetic corelamination stack according to a second aspect of the present inventionshowing the orientation of a fill part.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Those of ordinary skill in the art will realize that the followingdescription of the present invention is illustrative only and is notintended to be in any way limiting. Other embodiments of the inventionwill readily suggest themselves to such skilled persons from anexamination of the within disclosure.

Rotor bars used in various prior art asynchronous motors are mostlyelongate in that they are long and generally thin, but they utilizedifferent rotor bar vertical axial cross-sectional patterns dependingupon the use that the motor will be put to. For example, U.S. Pat. No.3,496,397 to Andresen teaches a number of such cross-sectional patternsand prior art designs. Similarly, U.S. Pat. No. 3,509,397 to Diamont,U.S. Pat. No. 2,899,618 to Noodleman, U.S. Pat. No. 2,971,106 toWestphalen, U.S. Pat. No. 1,986,368 to Schwarz, U.S. Pat. No. 3,987,324to Linkous, U.S. Pat. No. 3,683,493 to Begovich, and U.S. Pat. No.4,131,990 to Roach all teach various rotor bar cross sections applicableto a variety of uses and desired operating characteristics of theasynchronous motor. The invention taught herein is directed not to therotor bar's vertical axial cross-section but to a design for the rest ofthe rotor bar and is thus applicable with any or all of the foregoingrotor cross-section designs.

FIG. 1 shows a typical lamination 10 from a laminated magnetic core foran asynchronous motor. Lamination 10 is an end lamination (top orbottom) and comprises peripherally located and arcuately spaced apartslots 12 through which rotor bars are placed during assembly. FIG. 2shows a stack 14 of laminations 10 formed together with slots 12 alignedto form channels 16 through which rotor bars may be placed. Laminationsare preferably fabricated of a ferromagnetic material such as siliconiron as is well known to those of ordinary skill in the art.

Turning to FIG. 3, a rotor bar 18 according to a preferred embodiment ofthe present invention, comprises an elongate portion 20 and a headportion 22 which, when formed together, form a T-shaped rotor bar. FIG.4 shows a top view of head portion 22 of rotor bar 18 according to apreferred embodiment of the present invention. As can be seen in FIG. 4,head portion 22 is curved to conform to the outer circumference oflamination 10. As discussed above, the cross-sectional shape of theelongate portion 20 of rotor bar 18 may be any suitable shape dependingupon the use that the motor will be put to and the operatingcharacteristics desired.

An important aspect of the present invention is realized by using therotor bar heads 22 to form a conductive end ring for the rotor. FIG. 5shows a preferred arrangement for inserting rotor bars 18 so that theyinterleave to form end rings on both the top and bottom of the rotor.Rotor bars 18 are interleaved one with the head on top of the rotor, thenext with the head on the bottom of the rotor, the next with the head onthe top of the rotor, and so on, as shown. In FIG. 5, for clarity, theright-most rotor bar 24 is shown in front of and partially withdrawnfrom the magnetic core stack 14. The bar to its left is also shown infront of the stack. When inserted fully, end portion 26 will mate withadjacent head portion 28 and a similar arrangement will obtain at thebottom of the core stack 14 resulting in the formation of a conductiveend ring 30 (FIG. 7). As can be seen in FIG. 7, end ring 30 is formed ofalternating head portions 22 and mating end portions 26 of T-shapedrotor bars. By the use of the term "mating" or "mate" in reference tothe union of adjacent head portions and end portions at a surface, it isintended to include designs where the rotor bar cross sections have flatsides, curved sides, and complex sides (as shown, for example, at 26 inFIG. 7) which may also provide the capability of an interlocking"keying" between adjacent parts. Final electrical connection of thevarious constituents of the end ring is accomplished by brazing, weldingor soldering (or similar process) (collectively referred to as"electrically consolidating") to form a permanent electrical connectionamong the constituents. Presently preferred is a dip brazing process inwhich the end rings are immersed and heated in a brazing fluid whichconsolidates all of the abutting pieces of copper in the end rings. Suchprocesses are well known to those of ordinary skill in the art.

The end ring may preferably be machined down to a smooth circular outersurface 32 and then a containment ring 34 may be added in order toconfine the end ring 30 during high speed operation.

Preferably, the rotor bars 18 are fabricated of copper or an alloythereof with high electrical conductivity properties. Other similarmaterials could also be used, but relatively pure copper is presentlypreferred for its high electrical conductivity and relatively low cost.Containment ring 34 is preferably beryllium-copper (BeCu) which haslower electrical conductivity but is stronger and less ductile thancopper. Containment ring 34 may be slip-fit, as known to those ofordinary skill in the art, over circular outer surface 32 by cooling endring 30 to shrink it and heating containment ring 34 to enlarge it,placing containment ring 34 over circular outer surface 32, and thenallowing the materials to reach thermal equilibrium. While othermaterials could be substituted for BeCu, BeCu is presently preferredbecause its coefficient of thermal expansion is similar to that ofcopper, hence there are likely to be less radial differential forcesacting during normal operational thermal cycling between the end ringand the containment ring than if materials with dissimilar coefficientsof thermal expansion were used. This reduces the possibility of crackingand failure after repeated thermal cycling.

FIG. 6 shows a side view of the completed rotor assembly absent rotorshaft (not shown) which would be present in a working example and wouldpass through shaft apertures 36 (FIG. 1).

While it is presently preferred to use single piece pre-formed"T"-shaped rotor bars, many of the advantages of such rotor bars may berealized with lower tooling costs with the following construction.Straight rotor bars comprise an elongate central portion and a pair ofend portions. The straight rotor bars pass through the rotor barchannels of the magnetic core so that the straight end portions protrudeinto the volume that will form the end rings on each end of the rotorwhile the central portions of the rotor bars are retained within therotor bar channels. In this case, all rotor bars may be of constantcross-section but there is no requirement that they be, rather, such aconstruction is the simplest. Each rotor bar is attached to all itsadjacent clockwise and counterclockwise rotor bars by inserting a "fillpart" 38 made preferably of the same material as the rotor bars (again,preferably copper) into gaps 40 between clockwise and counterclockwiseadjacent rotor bar end portions 26 to form unconsolidated end rings.Here, the end portions 26 of rotor bars 18 extending into the volumewhich will be the end rings, together with the fill parts 38 form theunconsolidated end rings. In order to hold the assembly together untiland during brazing, the assembly may be clamped, tack welded, or, if arotor cross-section shape permits it, held together by the "keyed"interlocking effect of adjacent fill parts 38 and rotor bars 18 whichcan be seen in FIG. 8. Electrical consolidation is then achieved asdescribed above and, while the resulting structures have 2N braze jointsper end ring with N rotor bars, it is still an improvement over priorart rotors. One of skill in the art would prefer this mode ofconstruction where small numbers of rotor bars are to be constructed andthe tooling costs for the fabrication of one piece T-shaped rotor barsare prohibitively expensive. Where large numbers of rotor bars are to befabricated, one piece construction with stamping or a similar process islikely to be preferred.

While illustrative embodiments and applications of this invention havebeen shown and described, it would be apparent to those skilled in theart that many more modifications than have been mentioned above arepossible without departing from the inventive concepts set forth herein.The invention, therefore, is not to be limited except in the spirit ofthe appended claims.

What is claimed is:
 1. A rotor for an asynchronous electrical motor,said rotor comprising:a magnetic core formed from a stack of a pluralityof magnetic core laminations, said magnetic core laminations having aplurality of arcuately spaced apart rotor bar slots located at aperiphery thereof, said rotor bar slots aligned to form a plurality ofstraight arcuately spaced apart rotor bar channels through said magneticcore at said periphery thereof; a plurality of rotor bars, each saidrotor bar including a head portion and an elongate portion including anend portion, and said head portions formed to mate with adjacent endportions; said rotor bars inserted through said rotor bar channels in analternating orientation with said head portion of a second rotor barinterleaving and mating with clockwise and counterclockwise adjacent endportions of respective first and a third rotor bars; said end and headportions of said rotor bars electrically consolidated to form a pair ofelectrically conductive end rings disposed at opposite ends of saidmagnetic core, each said end ring consisting of alternating andinterlocking end and head portions of said rotor bars, each individualpair of end and head portions bonded together with a joint formed ofintervening braze material.
 2. A rotor according to claim 1 furthercomprising containment rings disposed about said end rings.
 3. A rotoraccording to claim 2 wherein said rotor bars are formed of copper andsaid containment rings are formed of BeCu.
 4. A rotor in accordance withclaim 1 wherein there are no more than one said joint per rotor bar ineach said end ring.
 5. A rotor for an asynchronous electrical motor,said rotor comprising:a magnetic motor core including a plurality ofmagnetic laminations having a plurality of arcuately spaced apart rotorbar slots at an outer periphery thereof, said plurality of arcuatelyspaced apart rotor bar slots forming a plurality of arcuately spacedapart rotor bar channels through said magnetic motor core at said outerperiphery; a plurality of T-shaped rotor bars, each having a headportion and an elongate portion having an end portion, said T-shapedrotor bars disposed in said rotor bar channels in alternating fashion toform end rings at opposite ends of said magnetic core, said end portionof each of said T-shaped rotor bars in contact with head portions of itsclockwise and counterclockwise adjacent neighbors disposed in clockwiseand counterclockwise adjacent rotor bar channels, each of said end ringsbeing electrically consolidated, each said end ring comprisingalternating and interlocking end and head portions of said rotor bars,each individual pair of end and head portions bonded together with ajoint formed of intervening braze material.
 6. A rotor according toclaim 5 further comprising containment rings disposed about said endrings.
 7. A rotor according to claim 6 wherein said rotor bars areformed of copper and said containment rings are formed of BeCu.
 8. Arotor in accordance with claim 5 wherein there are no more than one saidjoint per rotor bar in each said end ring.
 9. A rotor for anasynchronous electrical motor, said rotor comprising:a magnetic core,said magnetic core formed from a stack of a plurality of magnetic corelaminations, said magnetic core laminations having a plurality ofarcuately spaced apart rotor bar slots located at a periphery thereof,said rotor bar slots aligned to form a plurality of straight arcuatelyspaced apart rotor bar channels through said magnetic core at theperiphery thereof; a plurality of straight rotor bars, each said rotorbar including a first end portion and a second end portion with anelongate portion therebetween; said rotor bars extending through saidrotor bar channels so that said first and second end portions extendbeyond said rotor bar channels leaving gaps between clockwise andcounterclockwise adjacent rotor bars; fill parts disposed in said gapsand mating with clockwise and counterclockwise adjacent rotor bar endportions to form end rings at opposite ends of said magnetic core, eachof said end rings being electrically consolidated, each said end ringcomprising alternating and interlocking end and head portions of saidrotor bars, each individual pair of end and head portions bondedtogether with a joint formed of intervening braze material.
 10. A rotoraccording to claim 9 further comprising containment rings disposed aboutsaid end rings.
 11. A rotor according to claim 10 wherein said rotorbars are formed of copper and said containment rings are formed of BeCu.12. A rotor according to claims 1, 2, 3, 5, 6, 7, 9, 10 or 11 whereinsaid elongate portion of each said rotor bar has a predeterminedconstant vertical axial cross-sectional pattern.
 13. A rotor for anasynchronous electrical motor, said rotor comprising:a magnetic core,said magnetic core formed from a stack of a plurality of magnetic corelaminations, said magnetic core laminations having a plurality ofarcuately spaced apart rotor bar slots located at a periphery thereof,said rotor bar slots aligned to form a plurality of straight arcuatelyspaced apart rotor bar channels through said magnetic core at theperiphery thereof; a plurality of rotor bars, each of said rotor barsincluding a head portion and an elongate portion including an endportion, said head portions curved to conform with the outercircumference of said magnetic core laminations, and said head portionsformed to mate by interlocking with adjacent end portions, wherein saidelongate portion of each said rotor bar has a constant vertical axialcross-sectional pattern; said rotor bars inserted through said rotor barchannels in an alternating orientation with said head portion of asecond rotor bar interleaving with adjacent end portions of a first anda third rotor bar; said end and head portions of said rotor barselectrically consolidated to form electrically conductive end rings atopposite ends of said magnetic core, each said end ring comprisingalternating and interlocking end and head portions of said rotor bars,each individual pair of end and head portions bonded together with ajoint formed of intervening braze material; and containment ringsdisposed about said end rings, wherein said rotor bars are formed ofcopper and said containment rings are formed of BeCu.
 14. A rotor for anasynchronous electrical motor, said rotor comprising:a magnetic motorcore including a plurality of magnetic laminations having a plurality ofarcuately spaced apart rotor bar slots at an outer periphery thereof,said plurality of arcuately spaced apart rotor bar slots forming aplurality of arcuately spaced apart rotor bar channels through saidmagnetic motor core at said outer periphery; a plurality of T-shapedrotor bars, each having a head portion and an elongate portion having anend portion, wherein said head portion is curved to conform with theouter circumference of said magnetic core laminations, said T-shapedrotor bars disposed in said rotor bar channels in alternating fashion toform end rings at opposite ends of said magnetic core, said end portionof each of said T-shaped rotor bars in contact and interlocking withhead portions of its clockwise and counterclockwise adjacent neighborsdisposed in clockwise and counterclockwise adjacent rotor bar channels,each of said end rings being electrically consolidated, wherein saidelongate portion has a constant vertical axial cross-sectional pattern,each said end ring comprising alternating and interlocking end and headportions of said rotor bars, each individual pair of end and headportions bonded together with a joint formed of intervening brazematerial; and containment rings disposed about said end rings, whereinsaid rotor bars are formed of copper and said containment rings areformed of BeCu.
 15. A rotor for an asynchronous electrical motor, saidrotor comprising:a magnetic core, said magnetic core formed from a stackof a plurality of magnetic core laminations, said magnetic corelaminations having a plurality of arcuately spaced apart rotor bar slotslocated at a periphery thereof, said rotor bar slots aligned to form aplurality of straight arcuately spaced apart rotor bar channels throughsaid magnetic core at the periphery thereof; a plurality of straightrotor bars, each said rotor bar including a first end portion and asecond end portion with an elongate portion therebetween, said elongateportion having a constant vertical axial cross-sectional pattern; saidrotor bars extending through said rotor bar channels so that said firstand second end portions extend beyond said rotor bar channels leavinggaps formed by clockwise and counterclockwise adjacent rotor bars; saidfirst portion having a shape curved to conform with the outercircumference of said magnetic core laminations; fill parts disposed insaid gaps and mating with clockwise and counterclockwise adjacent rotorbar end portions to form end rings at opposite ends of said magneticcore, each of said end rings being electrically consolidated, each saidend ring comprising alternating and interlocking end and head portionsof said rotor bars, each individual pair of end and head portions bondedtogether with a joint formed of intervening braze material; containmentrings disposed about said end rings, wherein said rotor bars are formedof copper and said containment rings are formed of BeCu.